Unless Expression Parsing


#1

I just came across an unexpected parsing problem:

x = 3 unless true # parses just fine, x = nil

p(3 unless true) # parser chokes on unless

p((3 unless true)) # OK, ‘nil’ is printed

This seems like a bug in the parser rather than a real
syntax problem, but maybe I’m missing something?

Gary W.


#2

On Apr 19, 9:53 pm, Gary W. removed_email_address@domain.invalid wrote:

Gary W.

in irb:

x = 3 unless true => nil

x => nil # x declared as a nil
object since it was never assigned an object
p 3 unless true => nil

p (( 3 unless true)) => nil # you’re asking for the result
of ‘3 unless true’, )

p nil => # no result since p did
nothing

3 unless true => # the result is nil but not
captured
so
p (3 unless true) => # no result since you are asking
p to print nothing

Note:

the above is equivalent to
unless true so ‘p 3’ never gets
interpreted and nothing happens
p 3
end


#3

On Apr 19, 2007, at 11:50 PM, bbiker wrote:

p (( 3 unless true)) => nil # you’re asking for the result
of ‘3 unless true’, )

if ‘x unless y’ is a valid expression then why is the extra set
of parenthesis needed when that expression is used as a method argument?

foo(x unless y) # syntax error
foo((x unless y)) # not a syntax error

This seems to be a problem with any of the statement modifier forms:
if, rescue, while, until.

Looking at parse.y it seems that the grammar differentiates between
arguments and statements. Adding the parens forces the statement to
become an expression.